From Wikipedia, the free encyclopedia

576i is a standard-definition
video mode used in (former) PAL and
SECAM countries. In digital
applications it is usually referred to as "576i", in analogue
contexts it is often quoted as "625 lines".[1]

The 576 identifies a vertical resolution of 576 lines,
and the i identifies it as an interlaced resolution.
The field rate (not to
be confused with the frame
rate), which is 50 Hz, is
sometimes included when identifying the video mode—i.e.,
576i50.

Its basic parameters common to both analogue and digital
implementations are: 575 scan lines of picture content, 25 frames
(giving 50 fields) per second. Also in analogue, 50 additional
blank lines for the sync pulse are added, resulting in 625 lines.
Analogue television signals have no pixels; they are rastered in
scan lines, but along each line the signal is continuous.

In digital applications, the number of
pixels per line is an arbitrary choice as long as it fulfils the sampling theorem. Values above ca 500
columns are enough for conventional broadcast television; DVB-T,
DVD and DV allow better values such as 704 or 720.

Contents

Baseband
interoperability (analogue)

Spectrum of a System I (bands IV and V) television channel with PAL
or SECAM colour

When 576i video is transmitted via baseband (i.e., via consumer
device cables, not via RF), most of the differences between
the "one-letter" systems are no longer
significant, other than vertical resolution and frame rate.

Modulation for TVRO
transmission

576i when it is transmitted for TVRO viewing is transmitted substantially
differently from terrestrial transmission.

Full transponder mode (e.g., 72 MHz)

Luma signal is FM modulated, but with a 50 Hz dithering signal
to spread out energy over the transponder

Chroma is phase modulated

An FM subcarrier of 4,50, 5.50, 6.0, 6.50 or 6.65 MHz is added
for mono sound

Other FM subcarriers (usually 7.02, 7.20, 7.38, 7.56, 7.74 and
7.92 MHz) are added for a true stereo service and can also carry
multi-lingual sound and radio services. These additional
subcarriers are normally narrower bandwidth than the main mono
subcarrier and are companded using Panda 1 or similar to preserve
the signal to noise ratio

Data subcarriers may also be added

Half transponder mode (e.g., 36 MHz)

All of the above is done, but signal is bandwidth limited to 18
MHz

The bandwidth limiting does not affect audio subcarriers

Baseband
interoperability (digital)

In digital video applications, such as DVDs and digital
broadcasting, colour encoding is no longer significant; in that
context, 576i means only

576 frame lines

25 frames or 50 fields per second

Interlaced video

PCM audio (baseband)

There is no longer any difference (in the digital domain)
between PAL and SECAM. Digital video uses its own separate colour
space, so even the minor colour space differences between PAL and SECAM become moot in the digital domain.

Use
with progressive sources

When 576i is used to transmit content that was originally
composed of 25 full progressive frames per second, the odd field of
the frame is transmitted first. This is the opposite of NTSC. Systems which recover
progressive frames, or transcode video should ensure that this
field order is obeyed, otherwise the recovered frame will consist
of a field from one frame and a field from an adjacent frame,
resulting in 'comb' interlacing artifacts.

576i
speed-up

Motion pictures are typically shot on film
at 24 frames per second. When telecined and played back at 576i25's standard
of 25 frames per second, films run 4% faster. This also applies to
most TV series that are shot on film or digital 24p.[2] Unlike
480i30's telecine system, which uses 3:2 pulldown to convert the 24
frames per second to the 480i30 frame rate, 576i results in the
telecined video running 4% shorter than the original film as well
as the equivalent 480i30 telecined video.

Depending on the sound system in use, it also slightly increases
the pitch of the soundtrack by 70.67 cents (0.7067 of a semitone). More recently, digital conversion
methods have used algorithms which preserve the original pitch of
the soundtrack, although the frame rate conversion still results in
faster playback.

There also exist conversion methods that can convert 24 frames
per second video to 25 frames per second with no speed increase,
however image quality suffers when conversions of this type are
used. This method is most commonly employed through conversions
done digitally (i.e. using a computer and software like VirtualDub), and is
employed in situations where the importance of preserving the speed
of the video outweighs the need for image quality.

Some movie enthusiasts prefer 576i speed-up over 480i30's 3:2
pulldown, because the latter results in telecine judder, a visual distortion not
present in 576i speed-up video.[3]
This is not an issue on modern upconverting DVD
players and PCs, as they play back
23.97frame/s-encoded video at its true frame rate, without 3:2
pulldown.

Software which corrects the speed-up is available for those
viewing 576i DVD films on their computers, WinDVD's "PAL TruSpeed" being the most
ubiquitous. However, this method involves resampling the
soundtrack(s), which results in a slight decrease in audio quality.
The echo/audio balance issue can be resolved by re-adjusting the
playback pitch (located in the Audio Effect tab) from normal to low
and back to normal again.

See also

References

^DVDLard states "the
majority of authorities on the subject favour 576i over NTSC for
DVD playback quality". Also DVD reviewers often make mention
of this cause. For example, in his 576i vs. NTSC article,
the founder of MichaelDVD says
"Personally, I find {3:2 pulldown} all but intolerable and find
it very hard to watch a movie on an NTSC DVD because of it."
In the review of Frequency, one
of his reviewers mentions "because of the 3:2 pull-down
artefacts that are associated with the NTSC format (…) I prefer
576i pretty much any day of the week".

This table
illustrates total horizontal and vertical pixel resolution via box
size. It does not accurately reflect the screen or pixel shape
(aspect ratio) of these formats, which is either 4:3, or
16:9.